As image position sensitive devices for use in optical measurement of position, distance, movement, deformation, etc., there have been widely used semiconductor position sensitive devices(PSD) in which a resistive layer is provided on a photo sensitive layer. In the PSD, photo current is generated under a photo image projected on the photo sensitive layer. The photo current is transmitted through the resistive layer to opposite output electrodes. And the photo current is outputted from the opposite output electrodes. The intensities of the output currents are detected and the center of the intensities of the currents is calculated to determine the position of the photo image. The prior arts are disclosed, for examples, in G. P. Petersson and L. E. Lindholm, "Position sensitive light detectors with high linearity", IEEE J. Solid-State Circuits, vol. SC-3, pp. 392-399, 1987, D. J. W. Noorlag and S. Middelhoek, "Two-dimensional position sensitive photodetector with high linearity made with standard IC-technology", IEEE J. Solid State and Electron Dev., vol. 3, pp. 75-82, 1979, and D. J. W. Noorlag "Quantitative Analysis of Effects Causing Nonlinear Position Response in Position-Sensitive Photodetectors", IEEE Trans. Electron Devices, vol. ED-29, No. 1, pp. 158-161, 1982.
FIG. 5 shows a cross-sectional view of a typical prior art semiconductor position sensitive device. When light L is projected on a photo sensitive layer P, photo current is generated in the photo sensitive layer P. After passing through a resistive layer R, output currents I.sub.A and I.sub.B are outputted from the output electrodes T.sub.A and T.sub.B. Values of the output currents I.sub.A and I.sub.B are obtained by dividing the amount of the photo current in the resistance ratio of the resistive layer between the light incident position and the output electrodes T.sub.A and T.sub.B, respectively. Thus, value obtained by dividing the difference between the output currents (I.sub.A -I.sub.B) with the sum of the output currents (I.sub.A +I.sub.B) is proportional to the light incident position X.sub.D from the center. Accordingly, we can obtain information about the light incident position by introducing the output currents I.sub.A and I.sub.B into a signal processing circuit as shown in FIG. 6. In the circuit, the output currents are amplified by buffer amplifiers B.sub. 1 and B.sub.2. Next, the above operation is carried out by operational amplifiers A.sub.1 and A.sub.2 and a divider .div., and further, analog to digital conversion is carried out by an analog to digital converter A/D.
The precision of the prior art image position sensing method mainly depends on the precision of the analog signal processing means and the precision (resolution) of the analog to digital conversion. Improvement of the precision of the analog processing means has been almost impossible and even if it is realized it would be very expensive. This creates a big problem for improving the precision (resolution) of image position sensing by use of the semiconductor image sensing device.